The present invention relates, in general, to a method and to an apparatus for measuring motor currents in an electrical machine.
Electronically commutated electrical machines with permanent magnets, such as permanent-magnet synchronous motors, are common. Electrical machines of this kind are highly suitable for many applications, but at high rotation speeds have the disadvantage that the field attenuation required in this case leads to an increase in losses and therefore to a reduction in the degree of efficiency. In contrast to this, the field attenuation is achieved in a simple manner by reducing the excitation magnetic field in the case of electrical machines with external or hybrid excitation.
In the case of electrical machines with external excitation, the excitation magnetic field is generated with the aid of a separate field winding whereas, in the case of electrical machines with hybrid excitation, the magnetic field is generated both by permanent magnets and also with the aid of a separate field winding. The field winding serves to direct a static magnetic field toward a rotor body which collects the magnetic flux and guides the magnetic flux in the direction of the stator by means of the rotor poles. One advantage of machines with external or hybrid excitation is that the quantity of permanent-magnet material used is reduced and therefore the costs are lower in comparison to permanent-magnet machines.
The additional field winding typically requires increased circuit complexity. Furthermore, rapid and reliable measurement of the phase currents through the phase windings and of the field current is required in order to regulate and drive both the phase windings and the field windings, this resulting in additional expenditure. It is therefore necessary to optimize the expenditure on connecting the phase windings and the field windings and also on the current measurement operations.
WO 2012/089412 A2 describes a three-phase machine with a field winding which is connected to the star point of the three-phase machine and to the ground of the supply voltage, this already meaning a considerable simplification in the circuit complexity, in particular for the field winding.
DE 10 2006 052467 A1 further discloses how the phase currents of a three-phase machine can be determined with only one measurement resistor, that is to say one shunt.
Furthermore, WO 2013/000632 A2 discloses an extension of the known single-shunt current measurement operation to machines with external or hybrid excitation.
The object is therefore to provide means for determining the phase currents and the field current based on a current measurement operation with only one current measurement apparatus for an electrical machine with hybrid excitation. In the process, the method should be equivalent to or improved in respect of noise development and maximum possible driving level in comparison to the known methods for electrical machines without a field winding.